Georgi's book on Lie Groups is enough, but most of the group theory is explained in the physics texts. It is nice to learn group theory, but the mathematician's theory is more concerned with characters and root lattices, which are nice, but not essential in most of the bread-and-butter applications. The ALE classification is important in mathematical physics, but I think it is covered properly in the physics literature.
You don't need anything too special--- just the rudiments of Lie groups (it doesn't hurt to know group theory, though, it is just not essential). You can learn everything on your own from the QFT source and thinking it out--- there SU(2)/SU(3) cases are not too bad, and these are about as big as it gets. SU(5) and E8 require more sophistication, but are best covered in GUT papers and Green/Schwarz/Witten (for a great introduction to E8)
The modern algebra you probably want to learn is not group theory, but homological algebra, category theory, and Hopf algebra. These are covered well by Lang's algebra book, which is a graduate school staple in mathematics. It doesn't hurt to know everything in Lang--- it's well written, as everything by Lang--- although a little philosophically annoying for me, because it is so conservative in its set-theoretic appratus.
This post imported from StackExchange Physics at 2014-03-24 04:54 (UCT), posted by SE-user Ron Maimon